1. Field of the Invention
The present invention is directed to ultrafine and fine particles and methods for making and isolating such particles.
2. Discussion of the Related Art
Ultrafine particles are defined in the art as having diameters of about 100 nm or less. Such particles are therefore smaller than conventional powders, but larger than typical atom clusters. Ultrafine particles are of great interest due to their numerous applications, including use in the formation of ceramic and metal structures, conductive paths and/or conductive layers in electronic devices and the production of catalysts. For example, the use of ultrafine particles in forming ceramic and metal parts results in small grain size., thus providing the parts with optimal physical properties (e.g., strength and ductility). Also, in electronic devices, the small particles allow creation of finer conductor paths. Variations in processes used to produce ultrafine particles may also produce larger, so-called xe2x80x9cfinexe2x80x9d particles, which are defined as particles having diameters greater than 100 nm but less than 1500 nm. For many of the applications in which ultrafine particles are desired, fine particles may be equally useful.
It has been difficult, however, to obtain powder of ultrafine and fine particles without experiencing agglomeration into larger, less useful particles. Thus, those skilled in the art have attempted to isolate ultrafine and fine particles in a liquid suspension to prevent such agglomeration.
For example, U.S. Pat. No. 4,872,905 discusses a method of obtaining particles by utilizing a sputtering process and a liquid substrate. The metal particles generated from the target electrode encounter vapors of a heated liquid oil, are covered by the oil vapors, and are then captured by the liquid oil. A complex recovery process is required to obtain a usable end product. In particular, the liquid must be mixed with two solvents, such as kerosene and acetone, to thin out the oil and form a colloidal suspension. The acetone (or comparable solvent having a boiling point lower than both the other solvent and the oil) is removed by heating the solution, and the oil-covered particles then settle in the solution. This separation process may have to be performed up to four times. Moreover, prior to using the particles, the oil covering must be removed, for example by washing the particles in a solvent such as dioxane. Once the oil is dissolved, the particles will tend to agglomerate. Thus, while the method of this patent may offer a way of storing ultrafine or fine particles without agglomeration, it does not provide a means for producing isolated particles in a state that facilitates the actual use of the particles.
Another patent dealing with ultrafine and fine particles, U.S. Pat. No. 4,877,647, describes a method for obtaining a colloidal suspension of metal particles. Vaporized metal in a vacuum is captured by a solvent, which may be present as a gas or liquid. Typically, an external cooling set-up is provided, by which the solvent containing the captured metal atoms and atom clusters can be frozen to the interior of the vaporization vessel. The frozen matrix is slowly heated in the vessel to form a colloidal suspension of metal particles in the solvent. A large excess of solvent is required to obtain the suspension, however, at least 30 to 1000 parts by weight of solvent. Preferred metal loadings range from 0.02 to 0.09 molar. Above this level, the metal particles will tend to agglomerate and precipitate. Thus, ultrafine and fine particles produced according to U.S. Pat. No. 4,877,647 are difficult to utilize in many applications, because they cannot be used separately and distinctly from the large amount of solvent required. Moreover, reduction of the amount of solvent results in undesirable agglomeration, thereby destroying the particles usefulness.
The need therefore exists for methods of producing ultrafine and fine particles that remain isolated from one another, yet are in a state that facilitates handling and maximizes potential applications.
The present invention is directed to a method for isolating particles. The method involves the step of at least substantially encapsulating particles present as a highly dispersed colloidal suspension with an encapsulant material, such that the encapsulated particles remain independent and discrete upon separation from the suspension.
The present invention is also directed to a method for isolating ultrafine particles, including the steps of preparing a highly dispersed colloidal suspension of ultrafine particles of at least one metal in an organic solvent, adding to the suspension an encapsulant material such that the ultrafine particles are substantially encapsulated by the encapsulant material, and separating the encapsulated particles from the suspension.
The present invention is further directed to independent and discrete ultrafine and fine particles at least substantially encapsulated with an encapsulant material, wherein the encapsulant material is at least one compound selected from an amine, an ether, a thiol, a sulfide, a carboxylic acid, a hydroxy acid, a sulfonic acid, a polyhydroxy alcohol, an organosilane, a titanate, a zirconate, a zircoaluminate, a carboxylate, a sulfate, a sulfonate, an ammonium salt, a pyrrole, a furan, a thiophene, an imidazole, an oxazole, a thiazole, a pyrazole, a pyrroline, a pyrrolidine, a pyridine, a pyrimidine, a purine, a triazole, a triazine, and derivatives thereof, such that the particles remain independent and distinct.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.
The method of the present invention is applicable to particles of metal elements, nonmetal elements, inorganic compounds, and organic compounds, these particles capable of forming a highly dispersed colloidal suspension.
Solid elements such as boron, carbon, silicon and the Main Group metals, such as aluminum, beryllium, magnesium, etc., which are capable of forming atomic and cluster species in the gas phase, can form particles in a controlled fashion in a suitable medium to form colloidal suspensions.
Inorganic solid compounds, such as metal oxides, sulfides, selenides, tellurides, phosphides, antimonides, fluorides and other halide derivatives are known to form colloidal particles in suitable medium. Similarly, other inorganic compounds such as borides, carbides, nitrides and suicides form ultrafine: a particles in a controlled manner. Binary, ternary, and quaternary metal alloys, and the like, and intermetallic compounds can also form ultrafine particles using appropriate methods and conditions. Both organic and inorganic pigments, which may be produced by a variety of methods, require a great deal of physical and innovative effort to keep them well dispersed in a medium and produce them in a desired particle size and size distribution. The ability to keep such materials as discrete particles therefore enhances the efficiency of their production methods and improves their functional properties.
The initial step in producing isolated metal particles according to one embodiment of the present invention is formation of a highly dispersed colloidal suspension of metal particles in an organic solvent. One way to prepare such a suspension is described in U.S. Pat. No. 4,877,647, the entire disclosure of which is incorporated by reference herein.
The suspension may be prepared by first vaporizing a metal or metals to obtain metal atoms and atom clusters. The metal or metals preferably have an atomic number from 21 to 32, 39 to 50 and/or 72 to 82. The metal atoms and atom clusters are captured in a vaporized state in an organic solvent vapor. The atom- and atom cluster-containing solvent vapor is frozen or substantially frozen to form a matrix. The matrix is gradually warmed to room temperature without precipitating the captured metal atoms and atom clusters.
While all the metals listed above may be used, preferred metals for forming ultrafine particles are cobalt, nickel, copper, palladium, silver, platinum, gold, tin, lead, and mixtures thereof.
In such a method, the metal or metals are preferably evaporated under a vacuum with the resultant metal atoms and atom clusters condensed simultaneously with an organic solvent in vapor or liquid form to produce a frozen or substantially frozen matrix. A vacuum below 10xe2x88x922 torr, preferably below 10 xe2x88x924 torr, may be employed. Because an extremely cold surface is necessary to condense and freeze the metal atoms and metal clusters and the solvent to form a solid matrix, liquid nitrogen cooling may be used to freeze the matrix onto the interior of the evacuated chamber. It is preferred that the matrix of the metal atoms and atom clusters and solvent vapors be completely frozen, because a process involving a frozen or solid matrix is easier to control and will produce better results than a process involving a partially frozen matrix.
The solid matrix is preferably allowed to warm up slowly to room temperature, e.g., in a time period from one to four hours, to obtain the suspension. Alternatively, additional solvent that has been cooled to within about 15xc2x0 C. of its freezing point may be added to the frozen matrix, or the frozen matrix may be transferred to a vessel containing similarly cooled solvent. During such a transfer of the frozen matrix, or addition of solvent, it is preferable to keep the solvent vigorously agitated. This agitation speeds up the warming of the frozen matrix and helps to control the atom clustering process, thereby allowing attainment of particles of the desired size. This agitation also minimizes the amount of solvent required.
A preferred apparatus for preparing the suspension includes a rotary or static reactor assembly connected to a vacuum pump. The reactor assembly may consist of a flask equipped with a resistance heating source to vaporize the metal or metals being used. An electron gun, or the like, may be used instead of a resistance heating source. In addition, the reactor assembly may be equipped with one or more vaporizing sources. An inlet may be provided to introduce the solvent into the flask, preferably directing the solvent toward the flask walls to promote formation of the matrix. An external cooling set-up is preferably provided to condense and freeze the evaporated metal and the solvent on the flask walls.
At extremely low loadings of metal in the solvent, the metal is present as zero-valent solvated atoms or small atom clusters. The salvation phenomenon is characterized by a strong interaction between the atoms or atom clusters with the solvent, resulting in the shifting of electrons from the metal to the solvent molecules or vice versa. This shifting of electrons creates localized charges which provide charge stabilization to the suspension.
At higher loadings, the short-lived solvated metal atoms or atom clusters seek thermodynamic equilibrium by forming larger clusters, thus minimizing the free energy of interfaces. This leads to the formation of ultrafine or fine particles, depending on the amount of clustering. This phenomenon is highly controllable with the proper selection of metal-solvent combinations, solvent flow rate, concentration, temperature, and evaporation rate of the metal. Selection of these variables will optimally result in the formation of particles of the desired size. Particles produced by this method exist as highly dispersed colloidal suspensions characterized by high stability. A xe2x80x9cstablexe2x80x9d colloidal suspension is defined as a dispersion of colloidal particles that remain evenly distributed throughout the medium for several days, weeks or months without settling, agglomeration, or any change in the size of the discrete particles. The solvent prevents the metal particles from interacting or associating with each other, and for some solvent-metal pairs the suspension may remain stable indefinitely at ambient conditions.
The term xe2x80x9csolventxe2x80x9d as used herein encompasses organic liquids which are commonly referred to in the art as solvents, and which are able to form the colloidal suspensions discussed above. Polar organic solvents are preferred due to their greater ability to complex or solvate metal atoms and ago clusters in comparison to non-polar solvents, but any solvent which forms the desired colloidal suspension is suitable. Preferred dielectric constants for the solvent range from about 10 to about 55. Suitable solvents include ketones, alcohols, ethers and the like. Examples include, but are not limited to, acetone, methyl ethyl ketone, ethanol, propanol, dimethylformamide, triethylamine, benzaldehyde, acetaldehyde, tetrahydrofuran, dimethyl sulfoxide, and the like. Preferred solvents are acetone, tetrahydrofuran, methanol, ethanol, 1-propanol, 2-propanol, and the like. In the case of metal atoms, the solvent should be substantially non-reactive with the atoms at the conditions of the condensation in the reactor chamber, i.e., within 25xc2x0 C. of the solvent""s freezing point.
Similar methods for producing a colloidal suspension of metal particles in a solvent from vaporized atoms and atom clusters of metal are equally suitable for use in the present invention, as are all methods of forming suspensions of particles of non-metal elements, organic compounds, and inorganic compounds. Examples of such suspensions include, but are not limited to, hydrosols, organosols, and aerosols.
Common to all such methods of forming colloidal suspensions, however, is the need for a large excess of solvent, particularly in hydrosol and organosol systems, relative to the amount of recoverable and usable particles. The large amount of solvent required is problematic, in that it limits the usefulness of the particles for practical applications.
According to the present invention, an encapsulant material is added to a highly dispersed colloidal suspension of particles and substantially encapsulates the particles by forming an ionic or covalent bond with the particles, or by adsorbing onto the particles"" surfaces. The term xe2x80x9cencapsulate,xe2x80x9d as used herein, means the thermodynamically driven process in which the molecules of the encapsulant material coat or cover the particles, i.e., the molecules of encapsulant material at least partially cover the particles or take the form of a monolayer coating or sheath around the particles. Preferably, the entire surface of a particle is encapsulated. Optimally, the coating effectively prevents direct particle-particle interaction, thus preventing agglomeration of the particles. Instances of (a) adsorption of a single molecule of encapsulant material by two or more particles, and (b) weak interaction between adsorbed encapsulant material on separate particles, result in a weak connection between a majority of the encapsulated particles. The resulting interaction creates an open three dimensional network, and, as the networks of encapsulated particles grow, the networked particles begin to flocculate. The particles, however, due to the encapsulation, remain independent and discrete throughout this flocculating process. Preferably, the flocculent settles out of the suspension, thereby easing the recovery of the material. After settling out of the suspension, the material is easily re-dispersed by simple agitation. Upon separation from the suspension, the particles remain encapsulated as discrete entities at ambient conditions, but become available in a more concentrated and useful form.
The encapsulant material may be one or more compounds selected from an amine, an ether, a thiol, a sulfide, a carboxylic acid, a hydroxy acid, a sulfonic acid, a polyhydroxy alcohol, an organosilane, a titanate, a zirconate, a zircoaluminate, a carboxylate, a sulfate, a sulfonate, an ammonium salt, a pyrrole, a furan, a thiophene, an imidazole, an oxazole, a thiazole, a pyrazole, a pyrroline, a pyrrolidine, a pyridine, a pyrimidine, a purine, a triazole, a triazine, and derivatives of any of these compounds. Of these materials, an amine, an ether, a thiol, a sulfide, a carboxylic acid, a hydroxy acid, a sulfonic acid, a polyhydroxy alcohol, or derivatives thereof are preferred for metal particles; an organosilane, a titanate, a zirconate, a zircoaluminate, a carboxylate, a sulfate, a sulfonate, an ammonium salt, or derivatives thereof are preferred for non-metal particles; and a pyrrole, a furan, a thiophene, an imidazole, an oxazole, a thiazole, a pyrazole, a pyrroline, a pyrrolidine, a pyridine, a pyrimidine, a purine, a triazole, a triazine, or derivatives thereof are suitable for metals and some non-metals, but are preferred for metals. An appropriate encapsulant material may be chosen by one skilled in the art based on the characteristics of the particles and solvent.
Preferred encapsulant materials for metal particles are one or more compounds selected from triethanol amine, ethylenediamine, oleic acid, malonic acid, hydroxyacetic acid, dimethyl sulfoxide, propylene glycol, hexanetriol, dioxane, diethylene glycol dimethyl ether, dimethylformamide, 1-(2-cyanoethyl)pyrrole, 3-(2-furyl)acrylonitrile, 3-thiophenemalonic acid, mercaptobenzimidazole, 2-mercaptobenzoxazole, 6-aminobenzothiazole, 3-(2-aminoethyl)pyrazole, 1-pyrrolidinebutyronitrile, 3-pyridineacrylic acid, 4,6-dihydroxypyrimidine, 6-mercaptopurine, 1-chlorobenzotriazole, 2,4,6-triallyloxy-1,3,5-triazine, and derivatives thereof. Particularly preferred compounds for metal particles are malonic acid, oleic acid, 1,2,6-hexanetriol, and triethanolamine.
Monolayer forming compounds known as self-assembled monolayers, such as alkanethiols, dialkyl sulfides, dialkyl disulfides, alcohols, amines, and carboxylic acids are useful encapsulant materials for both metal and non-metal particles. Preferred compounds of this group are undecanethiol and diundecyl disulfide.
Encapsulants for non-metal particles with hydroxylated surfaces, such as silica, silicon, alumina, and the like, include coupling agents such as organosilanes, titanates, zirconates, and zircoaluminates. Preferred encapsulants of this type are trimethylethoxysilane, isopropyltriisostearoyltitanate, and neoalkoxytrisneodecanoylzirconate.
Encapsulants of the amphipathic type, for example, surface active agents such as carboxylates, sulfates, sulfonates, and ammonium salts are also useful encapsulant materials for non-metals. Preferred amphipathic type encapsulant materials are sodium stearate, sodium cetyl sulfate, sodium diisopropylnapthalene sulfonate, and cetyltrimethylammonium bromide.
The encapsulant material may be mixed with a solvent prior to adding the encapsulant material to the suspension, or, similarly, the encapsulant material can be added to the suspension along with additional solvent.
The amount of the encapsulant material to be added to a certain suspension may be determined by xe2x80x9ctitrationsxe2x80x9d of about 4 to 8 aliquot samples of the suspension. To each sample of the suspension, a progressively increasing amount of the encapsulant material, either alone or in solution, is preferably added. After 1 to 6 hours, two layers are formedxe2x80x94a first xe2x80x9csedimentxe2x80x9d layer containing the isolated encapsulated particles and a second layer ranging from clear to a color near that of the starting suspension. The sample having a clear second layer, yet with the smallest amount of added encapsulant material, is used to determine how much encapsulant material should be added to the suspension. Theoretically, this amount is the minimum amount needed to provide complete surface coverage to all the particles in the suspension. The actual amount required further depends on the total surface area of all the particles present in the suspension and the nature of interaction between the particles and the encapsulant. In some aerosol systems, in situ encapsulation may be difficult to control and may not be fully effective in producing and maintaining discrete particles.
The flocculent containing the isolated encapsulated particles can be separated from the solvent of the suspension by any conventional means, such as decanting the clear solvent layer or separating the lower layer containing the particles using a separatory funnel or similar apparatus. The recovered particles retain the encapsulation, along with a small amount of excess encapsulant material and residual solvent. The recovered particles thus take the form of an xe2x80x9cink.xe2x80x9d This xe2x80x9cinkxe2x80x9d constitutes a concentrated, more useful form of ultrafine or fine particles, wherein the individual particles remain independent and discrete. In this xe2x80x9cinkxe2x80x9d form, the particles remain isolated indefinitely at ambient conditions in sealed containers and may also be handled in the open air. The amount of excess encapsulant material and solvent in the xe2x80x9cinkxe2x80x9d can be reduced by allowing the solvent to evaporate by blowing with a stream of nitrogen gas, or by other means known in the art.
In addition to providing a more concentrated and useful form of isolated ultrafine particles, the present invention also results in particles having a narrow particle size distribution. This is due to the controlled atom clustering and nucleation process involved, as well as the phenomenon wherein the solvated metal atoms or atom clusters seek thermodynamic equilibrium by forming larger clusters to minimize the free energy of interfaces. Stability is also imparted by the encapsulation, which minimizes particle-particle interaction, and thus agglomeration. Typically, ultrafine particles formed according to the present invention will have an average diameter ranging from 20 nm to 70 nm, depending on the particular metal or metals used and the conditions employed. Larger, xe2x80x9cfinexe2x80x9d particles produced by adjustment of the process conditions will typically have diameters ranging from 100 nm to 1500 nm.
The concentrated, encapsulated, and isolated particles made according to the present invention may be used for a variety of applications. When using the particles for coating a substrate, additives such as fluxing agents, as well as a variety of surface treatment techniques, may provide enhanced adherence of the coating to the substrate. Suitable substrates include ceramics, metals, glass, polymer films, and various fibers. The particle xe2x80x9cinkxe2x80x9d may also be used for metal joining applications such as electronic component attachment. When the xe2x80x9cinkxe2x80x9d is applied in coatings or metal joining, under conventional coating or metal joining conditions, the encapsulant will typically evaporate from the particles or will form a liquid that can be washed off, leaving the ultrafine particles to interact and form a continuous layer or coating. Also, fibers such as, for example, Kevlar(copyright), Nylon(copyright), and natural fibers may be dipped into a metal particle xe2x80x9cinkxe2x80x9d of the present invention to impart conductivity. In addition a metal particle xe2x80x9cinkxe2x80x9d may be used to form solder paste and solder ink, or other metal inks. Metal particles of the invention may also be used to obtain a desired grain structure in ceramics, metals, and metal/ceramic composites. Specifically, manipulation of forming and treating conditions, within the knowledge of one skilled in the art, can be used to obtain a variety of grain structures to optimize properties such as strength and ductility. Other uses will be apparent to those skilled in the art.